Harmonic currents present in a power distribution network can present significant problems, including power losses, overheating, resonances and over-voltages, operational instability, and radio frequency disturbances. Any electronic circuit which presents a non-linear load to the power source will inherently generate harmonic currents. Power thyristors, rectifiers, and switching-mode power supplies commonly used in data processing and telecommunications equipment are inherently non-linear and are a major cause of power supply degradation due to generation of harmonics. The technique of using phase shifting transformers to cancel the harmonics in power distribution systems is well known.
As is well understood, loads on a power distribution system are generally unknown prior to installation of the power distribution system. Even when initially predicted, the loads often change as equipment is added to or removed from the system. It is thus advantageous to be able to adjust phase shifting transformers in the field after initial installation.
U.S. Pat. No. 5,543,771 to Levin, which issued Aug. 6, 1996, teaches a phase shifting polygonal transformer or autotransformer for a three-phase electrical distribution system, in which the transformer output winding or autotransformer winding is composed solely of three main coils and three auxiliary coils, alternately interconnected in series. The outputs of the transformer/autotransformer are connected to taps in the coils offset from the connections between the coils. However, Levin does not provide field-adjustable phase shifting.
U.S. Pat. No. 6,169,674 to Owen, which issued Jan. 2, 2001, teaches a transformer for controlling harmonic currents by enabling different phase relationships to be set, and changed in the field, between devices being energized and the power source providing the energization. Owen teaches transformers of closed polygon configurations. This has particular application, for example, in canceling harmonics caused by multiple six-pulse variable frequency drives used for controlling connected three-phase induction motors that operate electric submersible pumps. The transformer used to achieve this has two winding groups, each with two sets of contacts at different phase relationships.
U.S. Pat. No. 5,343,080, which issued to Kammeter on Aug. 30, 1994, teaches a harmonic current filtering transformer that includes a three-phase input winding and at least two wye-connected three-phase output windings. The output windings are phase shifted relative to each other by an amount which causes harmonic currents generated by a non-linear load to magnetically cancel in the transformer core. Kammeter does not provide selection of different phase shifts in the field.
Consequently, aside from Owens transformer, which is complex to construct and install, and adapted to a single purpose, the prior art fails to teach an efficient and economical solution for harmonic cancellation, especially harmonics on a common supply bus which affect power losses in the distribution grid.
There therefore exists a need for a field-adjustable phase shifting transformer for canceling harmonic currents in a three-phase power system that facilitates transformer construction and field adjustability.